Cryptococcus neoformans is one of the most serious fungal pathogens affecting primarily immunocompromised individuals such as those with AIDS. It is a unicellular budding yeast which is surrounded by a thick polysaccharide capsule. The fungus is neurotropic and causes meningoencephalitis which is fatal unless treated. Considerable progress in the identification of genes associated with pathogenicity and virulence of C. neoformans has been made in this laboratory. During 1993-1997, three capsule genes have been identified and characterized as essential for virulence of the organism. During the past year, we have cloned the fourth gene, CAP10, which is also essential for capsule formation as well as virulence. This year we have conducted molecular dissections of the gene and constructed an expression plasmid using green flourescent protein (GFP) to localize the gene product. Sequence comparisons between genomic and cDNA fragment indicate the presence of 3 introns in the CAP10 gene. The gene encodes a putative protein containing 640 amino acids with a molecular weight of 73kD. The HA epitope-tagged Cap10 protein was about 73 kD which was predicted from sequence analysis. The acapsular mutant carrying genetic defect in the CAP10 gene was transformed with the CAP10 gene tagged with the GFP protein showed that the gene product is localized in cytoplasmic organelles. In addition, transcription of the CAP10 gene was found to be regulated by a transcriptional activator, STE12alpha, a homolog of Saccharomyces cerevisiae STE12 which is in the MAP kinase signal transduction cascade to pheromone response.